FIG. 5 shows an initial direct-current power supply circuit called linear AC adopter. In FIG. 5, a commercially available alter later voltage Vi input into the part between a AC input terminal 10 and 11 is converted to a prescribed voltage at a transformer 12, rectified at a full-wave rectifier 13, and smoothed at a smoothing condenser 14. Then, a direct-current voltage Vo and direct-current Io are supplied to a load 17 through direct-current output terminals 15 and 16.
In the simple and cheap power supply circuit of direct current called linear AC adopter shown in FIG. 5, when the input alternate-current voltage Vi fluctuates, the output voltage Vo varies according to the input voltage, such as V1, V2, and V3 show in FIG. 6. A particular problem is that when the output direct-current increases, the output voltage Vo tends to gradually decrease. Consequently, among loads 17 for commercially available electronic devices, many devices are designed to adjust to the property that the output voltage Vo decreases gradually with the increase of the output direct-current Io.
Recently, among direct-current power supply circuits with a relatively low output of 50 W or less, RCC (Ringing Choke Converter) type switching power supply circuits shown in FIG. 7 have been being used in place of those shown in FIG. 5.
In FIG. 7, when a direct-current voltage rectified an AC voltage is applied to a part between a plus side terminal 18 and a minus side terminal 19, a voltage of threshold level or less is applied immediately at a gate of a first switching element 22 with MOS•FET through a starting resistance. Then, a slight drain current runs at the first switching element 22, by which a voltage is generated at a magnetizing coil 25 of a fly-back transformer 24. A voltage is then induced at a drive coil 27. The voltage generated at the drive coil 27 is loaded on the gate of the first switching element 22 through resistances 31 and 33 and a condenser 34, by which a positive feedback loop is formed and the first switching element 22 turns on immediately at t1, as shown in FIG. 8 (a). At that time, a voltage Vds between the drain and source of the first switching element 22 becomes 0 as shown in FIG. 8 (a). With the on-state of the first switching element 22, a voltage approximately equivalent to a direct-current voltage applied to the part between the plus side terminal 18 and the minus side terminal 19 is applied to the fly-back transformer 24 and a voltage Vd determined by the coil-winding number ratio of a output coil 26 to the winding number of the magnetizing coil 25 as shown in FIG. 8 (d) is generated at the drive coil 27.
The voltage Vd generated at the drive coil 27 induces a start to charge up a condenser 28 through a resistance 31 and a light-sensitive element 46 of a photo-capra 32. When a voltage Vc charged at the condenser 28 becomes a wave shape as shown in FIG. 8 (e) and the voltage Vc reaches a voltage Vbe between the base and emitter of the second switching element 23 at t2, a base current runs at the second switching element 23 to turn on. With the on-state of the second switching element 23, the voltage between the gate and source of the first switching element 22 becomes approximate zero and the first switching element 22 thus turn off immediately. During the t1˜t2 period from the on-state to the off-state of the fist switching element 22, as shown in FIG. 8 (b), a current Id runs at the magnetizing coil 25.
When the first switching element 22 turn off at t2, simultaneously, a voltage inverting for positive or negative states to that of the magnetizing coil 25 side is generated at the output coil 26 of the secondary side. This voltage is rectified at a rectifier diode 40 and smoothed at a smoothing condenser 41 and then supplied to the load 17. At that time, the current Io running at the output coil 26 is shown in FIG. 8 (c). The current Io rapidly increases and subsequently decreases gradually toward zero until the rectifier diode 40 turns off. Simultaneously, at t3 when the rectifier diode 40 turns off, the inverted voltage Vd as shown in FIG. 8 (d) is generated at the drive coil 27 by residual magnetic flux inside the output coil 26 and the first switching element 22 then turns on again. During the time, the condenser 28 is charged up with the negative state by the voltage Vd inverting through the resistance 29 and a Zener diode 30. An output voltage is obtained by the repetitive oscillation operation.
The output voltage Vo detected at a resistance 48 is compared with a basic voltage V ref of the Zener diode 47. When the output voltage Vo exceeds the basic voltage V ref, the difference runs at light-emitting element 45 of the photo-capra 32. Because this then changes an impedance between a collector and emitter of a light receiving element 46 of the photo-capra 32, the outcome changes a time constant resulting from the resistance, the light receiving element 46 of the photo-capra 32 and the condenser 28, all of which are on the route for charging the condenser 28 during the on-state of the first switching element 22. Consequently, the condenser voltage Vc changes with the output voltage Vo. That is, higher the output voltage Vo, earlier the condenser voltage Vc reaches the Vbe, and conversely, lower the output voltage Vo, slower the condenser voltage Vc reaches the Vbe. When the condenser voltage Vc reaches the Vbe, the second switching element 23 turns on and the first switching element 22 turns off. The changed time constant then changes the time until the Vc reaches the Vbe and the on-state period of the first switching element 22 is then controlled to stabilize the output voltage Vo.
In FIG. 7, to prevent a high-spike voltage generated under a transient condition of the switching in the switching power source, a snubber circuit consisting of a diode 35, a condenser 36, and a resistance 37 is connected in parallel to the magnetizing coil 25.